Stupid Post on Stupid Leaks From Stupid Caps

[TC]

This seems so trivial that I hesitated to post it, but what the heck...

Whenever I purchase motor oil I automatically check and tighten the caps before I even get to the cash register. Years of routinely seeing wet bottles on the store shelves led to that, plus the desire to avoid an Exxon Valdez situation in my car trunk. Last December I bought 15 bottles of Pennzoil Platinum at AutoZone, and I can almost bet my mortgage that I checked the caps. The bottles sat for a half-year before I decided to sell them to a coworker the other day (as separate story). So I had the bottles in plastic grocery bags on the rear floorboard of my car for two days before removing them at work. I found that at least one of the bottles had leaked at least a cup of oil. Luckily most of it was trapped in the bag, but some of the oil found its way onto the carpet.

This puzzled me since I knew – or at least was ALMOST certain – that I had tightened them upon purchase. Later that day I checked the one bottle of Pennzoil I didn’t sell, but kept in the trunk of my car as “road oil,” a bottle that I KNOW I checked the cap on since it can only rest on its side where I stored it in the trunk, so it was mandatory to check that cap. And that cap was modestly loose as well! This surprised me since I KNOW I checked it. So a lesson was learned: I tightened all those bottle caps in winter when it was cold. When I sold the oil the other day, all the bottles were warm to the touch. Caps tightened in cold winter = looser caps the following warm summer, enough so that they may leak. Like I said, an almost trivial matter, but one that might save you from needing to shampoo your car or trunk carpet, so a worthwhile one nonetheless.

 

[eljefino]

Not a plastics expert but I think they heat the caps up and jam them directly on the bottles at the factory. If you look at the threads they're wedge shaped so they "click" down and on. This is how they manage to get it on with the serrated tamper resistant ring intact. A jerk could heat the cap with boiling water, gently lift it off, and refill a container with counterfeit oil if they were so inclined.

I'm almost concerned you're overtorquing your caps creating a weakness.

I have a nice reclined shelf in my trunk caused by a wheelwell that I prop my oil bottles on. They sit at about a 45 degree angle.

 

[TC]

Interesting....I always wondered how the tamper caps were installed at the plant. Makes sense, if that's the method used. The tightness of a cap isn't of much significance, of course, unless the bottles are transported or otherwise tip over. When sitting on a shelf, no problem, of course. Some Lubro-Moly jugs I bought have plastic seals (flat circular plugs) inside the top of the spout that act as both tamper plugs and leakage prevention. One could remove the jug caps completely and store the jugs upside-down if they wanted, with no leakage, so long as the tamper caps were in place. It’s actually pretty foolproof. Not sure why that isn't more common with other brands. On a related note, what's up with all the routinely "wet" motor oil bottles in the stores? Doesn't that automatically suggest a defective packaging design? Heck, I seem to recall that even the old cardboard-and-metal oil cans from the 70s could seal in oil fairly reliably, although maybe I’m wearing rose-colored glasses when I recall that.

 

[mechtech2]

I worked at Illinois Bottle/Arrow plastics. The bottle threads are 'J' threads, they grab better with that flat bottom for slippery plastics.
Heat expansion, plastic deterioration, internal air pressure, the type of seal used, and vibration induced loosening are all possible causes of a leak over time in a trunk.